Every radio amateur should build at least one antenna to prove to
themselves they can improvise in an emergency. One of the easiest and
quickest antennas to build is the Vertical Bazooka, which is made entirely
from RG-8U coaxial cable.

The plan for a Vertical Bazooka is in the drawing above.
Copy the image to your own hard disk and you are all set. The design and
image are public domain so feel free to copy them and give them out as you
wish.

Beyond being extremely easy to build,
these antennas have several nice features to recommend them:

Total cost under
$10.00cdn

No coils needed.

No ground radials.

SWR under 2:1 across
several megahertz

They are surprisingly
rugged antennas.

Can be built as part
of your feedline

Can be coiled up and
put in your car trunk.

Performance like
regular half wave antennas.

The antenna itself is an off-center fed vertical dipole made
by flipping a quarter wavelength of braid back over the outside of the
coax. The center lead of the coax forms one half of the dipole and the
braid forms the other. The resulting antenna has a low radiation pattern
and an impedance of 55 - 60 ohms.

One of the more interesting features is the braid itself. By
folding an electrical quarter wavelength of braid back over the insulated
coax we are forming both a dipole and a "bazooka" (sleeve) balun, a
coaxial 1:1 balun that greatly reduces feedline radiation. This makes the
Vertical Bazooka a good choice for use on apartment balconies.

The braid side of the dipole ends up considerably shorter
than the top element because of the interaction between the braid and the
coax. The outer braid couples with the inner braid to form the balun and
is thus affected by the velocity factor of the coax. Note on the plans
that I've compensated for the length difference. Also note the formulas
given will start you off long so you have to trim for the best SWR.

You can use the formulae in the diagram above to create
Vertical Bazooka dipoles for any frequency. However; practical
considerations of mounting and support tend to limit it to the span from
10meters (28mhz) to 70cm (440mhz). Below about 25mhz the housing tube
becomes very long and can be difficult to support. Above about 450mhz you
are pushing the frequency limits of the coax which are affected by the
lower braid section of the dipole.

Power handling with good quality coax should be 100+ watts.
However; if you plan to run more than 50 watts on a regular basis, as a
precautionary measure, you should add a couple of layers of electrical
tape or heat shrinkable tubing between the braid and the outer insulation
of the coax.

The Vertical
Bazooka is a fun and easy antenna that can be built in an
afternoon.

Building The
Antenna

My weekend project was to build a Vertical Bazooka for 2
metres. I was easily able to build the antenna and get it set up on my
balcony in a day, including several coffee breaks as I went. I had more
trouble getting some weight for the base I used than I did building the
antenna.

One preliminary step I would strongly recommend if you are
working from an enclosed location such as an attic or a balcony, is to
spend some time with a handy talkie and find out where you are going to
get the best results. Position the finished Vertical Bazooka in your
personal "hot spot". On my balcony this was just a bit West of
center

.

On the left
you see all the parts needed to build a Vertical Bazooka antenna. The
larger circle of cable is RG-8U coax which will become the antenna. The
inner coil is my feedline made of 4 wavelengths of RG-8X coax. At the back
of the picture you see the grey plastic tube, a piece of electrical
conduit that will hold the finished antenna.

NOTE: IN THIS PROTOTYPE, GRAY PVC WAS USED. IT IS HIGHLY
RECOMMENDED THAT YOU USE WHITE PVC. GRAY HAS MUCH MORE ATTENUATION TO RF
THAN WHITE!

The first step is to decide how you want to mount the
antenna. It is important to have enough of a gap between the bottom of the
coax shield and the mounting structure. If this distance isn't at least
1/8 of a wavelength it is very likely the SWR will be affected when you
mount the antenna.

If it's going on top of a tower, you can simply make the
plastic housing tube as long as you want, leaving plenty of extra for the
mounting gap. If you are doing a balcony mount a little figuring will have
to be done to ensure the antenna will fit between your balcony and the one
above it, with some clearance above.

For my balcony mount I decided on 160cm. Mounting this on a
75cm piece of pipe on a round base gives me lots of mounting gap
(certainly more than the minimum) but doesn't make the antenna so tall
that it would not work between my balcony and the one above. I actually
worked it out so the bottom of the braid would be a couple of centimetres
above the steel top rail of the balcony, which seems to have been a
workable plan.

Having decided the lengths of things I then cut
the tube and temporarily assembled it to the base. Next I put a PL259
connector on the the RG8-U coax and cut it to fit in the tube. Obviously
you want the antenna to be the full length of your tube so be sure to
allow a couple of extra inches when cutting. At this point I had a piece
of RG-8U, the right length, with one connector installed, as you can see
in the picture. This will become my antenna.

The next step was to measure down 1/4 wavelength from the
open end of the antenna so I could cut away the outer insulation and
expose the braid. This distance is determined by the formula:

Being extremely careful not to damage the braid,
I made a cut around the outer insulation of the coax at this distance and
cut along it's length. This allowed me to remove the outer insulation,
leaving me with the braid exposed as shown in the picture to the
left.

Now comes the fun part. Without unraveling it, I had to push
the braid back over the still-insulated part of the coax, reversing it on
the cable. In other words the braid came up inside the outer insulation
and then went back down on the outside.

Fortunately this isn't all that hard to do. The braid on
coax works like "Chinese Handcuffs" in that when you push back on it it
will expand in diameter. I worked from the open end back, getting it all
loose and a little bit expanded, then I just grabbed it at the bottom
(where the insulation ends) and rolled it back over itself. This took
about 10 seconds, once I got the hang of it.

The larger picture to the left shows the antenna with the
braid flipped back. At the right is a close-up of the feedpoint (where the
center and braid go opposite directions) showing how the braid is flipped
back over the outer insulation.

Now I hit a little snag. When you loosen the braid as I did
it gets springy and very difficult to keep in place. I finally ended up
wrapping tape around the bottom to hold it in place while I trimmed. I
then found I could could trim it very easily with cutter pliers.

For your
first cut on the braid use the formula

(7500 / frequency)
X velocity factor = length (in
centimetres)

It is very important that you know the velocity factor of
the RG-8U coax you are using for the antenna. If you have the .80 type,
cutting for .66 will make it too short. If you have the .66 cutting for
.80 could cost you a lot of hours trimming it down. So, be sure
first.

My coax was the .66 velocity factor type so substituting
into our formula we get:

(7500/146) X .66 =
33.91 cm (13.35 inches)

This should get the length of the braid very
close to the correct length and very little trimming should be
needed.

On the right is a picture of the antenna taped to the side
of it's housing just before I took it outside for tuning. Note the tape at
the bottom of the braid, to keep it from springing back.

Trimming the top section for best SWR is easy. The final
length of the upper section (center lead) will vary according to the
dielectric effect of the rather thick insulation on it. So I cut very
slowly taking no more than a centimetre, often just a couple of
millimetres at a time.

After each cut I would check the SWR at 144.000mhz and
148.000mhz. Getting them equal means the antenna is the right length. If
the low frequency is better the antenna is too long. If the high frequency
is better the antenna is too short. So you know when to stop
cutting!

Trimming the braided shield is a bit tricky. Each cut has a
large effect and tends to affect the SWR equally all across the band. If
you find you can't get the SWR under 1.5 to 1, you can try a small cut on
the braid, no more than a couple of millimetres, and see what happens. If
the SWR improves try it again... if not, stop where you are.

You should note that you won't likely get a 1:1 SWR out of
this antenna. It's feedpoint impedance is closer to 60 ohms, so anything
under 1.5:1 is good.

I discovered through experimentation that I actually wanted
the antenna just a tiny bit short. Inserting it into it's tube had an
effect like lengthening it slightly, probably due to the dielectric effect
of the plastic. So the first cut you make that's too short, stop and test
it in the tube... it's likely to be the exact length you need.To the left is a
shot of the antenna all ready to be assembled and put to work. Note that I
have put heat shrinkable tubing over the braid part of the antenna. This
is to prevent it from moving after assembly, vinyl tape should work as
well. If the braid moves inside the housing, the SWR will be
affected.

There were now only
a couple of minor details left:

The first task was keeping the antenna from
sliding out of the plastic tube that was to be it's home. This was easily
solved by making a small hole in the tube and using a wire tie to clamp
the antenna into place.

The last detail was some waterproofing which I handled by
putting a chair leg cap over the top of the tube. You can get these at
most hardware stores in packs of 4. The nylon ones are really tough and
work well.

And finally once it was all together I put it on
an old fan base I had sitting around and added a little weight to keep it
all in place. It would have been better to clamp it to the railing, but at
the time I didn't have my landlord's permission. The final assembly,
sitting on my balcony is shown on the left.

The
antenna is almost invisible from the ground. It looks like a mop handle or
broomstick, and not at all like an antenna.

The SWR is under 1.5 across the entire 2 meter band. It
measured 1.3 at each end and 1.1 in the middle during my tests; not
perfect but totally usable.

Some Suggested
Modifications

Since originally
posting this article I've received a few emails with suggestions for
improving the design. Here are a couple of the better ones...

USE WHITE PVC INSTEAD OF GRAY. MUCH LESS RF
ATTENUATION

Use copper tube for the
lower section.

Instead of using the braid itself, a section of copper
tubing could be used to make the lower section. In this case cut the braid
leaving only a short stub that would be soldered to the copper tube. The
length is the same as for the braid version, but the tube is more
dimensionally stable. You would trim it with a pipe cutter. (I've not
tested this version, so additional feedback would be appreciated)

Use several pieces
of wire to replace the lower section.

In this modification you replace the shield section with 3
or 4 runs of insulated 16ga lamp wire. To do this cut the braid and
twist it to form a short stub for soldering. Make a ring of wire to fit
around the coax, attach the wires to the ring and solder to the shield
braid. The wires are then secured to the sides of the coax with heat
shrink tubing. To adjust this new bottom section, clip bits off the ends
of the wires. This modification has the advantage that you can still
roll the antenna up, if used for an emergency antenna.

More mods and
improvements will be posted as they come in. To offer a suggestion click
the "Feedback" link at the bottom of this page.

Putting It To
Work

In checking with a number of people around my area, up to 30
miles away, signal reports have been fair, not braggable. Most reports are
about the same as I would expect for a quarter wave, all were far better
than for my handy talkie. The performance is adequate, so the overall
project is what I'd call a success.

Reports from a couple of other Amateurs who have put
Vertical Bazookas on mast pipes are better than those from my balcony.
This I think is to be expected since the operating environment on my
balcony, surrounded by sheet metal, is far from ideal.

Making an emergency version of this antenna is simple. Just
don't put it in the tube. You can then coil it up and put it in your
emergency communications kit. Add a little coax and some nylon cord on the
top end and you can string it over a tree branch or hang it from a
clothesline while you need it.

UPDATE! 5-2011 courtesy of
W6JMF

I built this, and mounted it
to one of those fiberglass poles they sell for bike flags. Used half inch
shrink to hold it on. Does tend to flex in the wind, but seems happy
at 70+ mph. I built a mount out of aluminum angle and a couple of
the big Harbor Freight magnets. Looks a little hillbilly when you get
close. The best match I could get was 1.3:1, but it's very
broadbanded and stayed under 2:1 for +-5Mhz. The whole point of the
exercise was to find something better than a mag mount that didn't depend
on the car body for a ground plane.

message starts......

I tried building it with
lmr 400 (corrected for VF .85) Couldn't get it totune.
Tried again with rg 8x. Finally got close enough to understand
whatshappening. Using a MFJ259 to sweep, I was able to identify
that this typeof antenna has horrible feedline coupling to anything
metallic in a fivemile radius (perhaps slightly overstated), making it
unsuitable withoutcorrection for cars with metal bodies. It was
only after I put 2 passes ofthe coax through a good sized snap on
toroid (just below the braid), that itwould repeatably tune. My
experience in tuning it was to trim the radiatorportion to get
the dip in the middle of the band, then tweak the shieldlength for
minimum vswr.

Really need to remind people that
a choke is not optional. Even if
you'rehanging this from a tree, you need to decouple that feedline if
you expect adecent
pattern.